There are plenty of unit/regression testing tools for Emacs, and even some for functional testing. What Emacs is missing though is a really good testing framework for integration testing. This is where Ecukes comes in.

Cucumber is a great integration testing tool, used mostly for testing web applications. Ecukes is Cucumber for Emacs. No, it's not a major mode to edit feature files (see feature-mode for that). It is a package that makes it possible to write Cucumber like tests for your Emacs packages.